A cellular telecommunications system is a communication system that is based on use of radio access entities and/or wireless service areas. The access entities operate over respective coverage areas that are typically referred to as cells. Examples of cellular telecommunications systems include standards such as the GSM (Global System for Mobile communications) or various GSM based systems (such as GPRS: General Packet Radio Service), AMPS (American Mobile Phone System), DAMPS (Digital AMPS), WCDMA (Wideband Code Division Multiple Access), TDMA/CDMA (Time Division Multiple Access/Code Division Multiple Access) in UMTS (Universal Mobile Telecommunications System), CDMA 2000, i-Phone and so on.
In a cellular system, a base transceiver station (BTS) provides a wireless communication facility that serves mobile stations (MS) or similar wireless user equipment (UE) via an air or radio interface within the coverage area of the cell. As the approximate size and the shape of the cell is known, it is possible to associate the cell to a geographical area. Each of the cells can be controlled by an appropriate controller apparatus.
Elements of the cellular network can be employed for provision of location information concerning a mobile station and the user thereof. More particularly, the cells or similar geographically limited service areas facilitate the cellular telecommunications system to produce at least a rough location information estimate concerning the current geographical location of a mobile station, as the cellular telecommunications system is aware of the cell with which a mobile station currently associates. Therefore it is possible to conclude from the location of the cell the geographical area in which the mobile station is likely to be at a given moment. This information is available also when the mobile station is located within the coverage area of a visited or “foreign” network. The visited network may be capable of transmitting location information of the mobile station back to the home network, e.g. to support location services or for the purposes of call routing and charging.
A location service feature may be provided by a separate network element such as a location server which receives location information from at least one of the controllers of the system. If no further computations and/or approximations are made, this would give the location to an accuracy of one cell, i.e. it would indicate that the mobile station is (or at least was) within the coverage area of a certain cell.
However, the accuracy of the location determination may be improved by utilising results of measurements which define the travel time (or travel time differences) of the radio signal sent by a mobile station to the base station. More accurate location information may be obtained through e.g. by calculating the geographical location from range or range difference (RD) measurements. All methods that use range difference (RD) measurements may also be called TDOA (time difference of arrival) methods (mathematically RD=c*TDOA, wherein c is the signal propagation speed). Observed time difference (OTD), E-OTD (Enhanced OTD) and TOA (time of arrival) are mentioned herein as examples of technologies that are based on the RD measurements.
The difference between the TOA (time of arrival) and the E-OTD is in that in the TOA the mobile station sends the signal and network makes the measurements, whereas in the E-OTD the network sends the signals and the mobile station measures them. The mobile stations are provided with appropriate equipment and software to provide information on which the positioning of the mobile station can be based on. The mobile station may communicate the information via the base station to an appropriate network element that may use the information in a predefined manner.
It is also possible to form RD measurements based on other sources, e.g. from GPS (Global Positioning System) pseudo-range measurements.
Therefore it is evident that there exist a variety of techniques for determining the position of the mobile station within a cell. In many of these systems, for example TDOA, the position measurement signals are received by LMUs (Location Measuring Units) from the MS and then sent to a SMLC (Serving Mobile Location Centre) that calculates the location of the MS.
Accurate positioning information is particularly required for emergency services, so that an emergency service provider is able to determine an accurate estimate of the current location of a MS.
The United States Federal Communication Commission (FCC) has mandated that wireless service providers have to implement location technologies that can locate wireless phone users who are calling to emergency numbers. In particular, the FCC's Third Report and Order (FCC 99-245) specifies the following standards for Phase II location accuracy and reliability:                For network based solutions:—100 meters for 67% of calls, and 300 meters for 95% of calls;        For handset based solutions:—50 meters for 67% of calls, and 150 meters for 95% of calls.        
The ANSI (American National Standards Institute) specification TIA/EIA/J-STD-036-A having the title “Enhanced Wireless 9-1-1 Phase 2” defines for ANSI 41 and GSM systems how initial and updated location information is determined and routed to a PSAP (Public Safety Answering Point) during an emergency call. However, this routing is based on cell level location information, wherein a MSC (Mobile Switching Centre) routes all emergency calls from certain cells to a particular PSAP.
However, this solution is not optimised in situations in which there are two or more PSAP service areas that over lap with the coverage of a single cell, and the problem becomes even more acute when the cells are setup to have large coverage areas.
The present invention is related to allowing emergency calls to be routed based on geographical position within a cell.
Therefore there is a need for an improved manner of routing emergency calls based on the geographical location.